Cut the flank steak into thin slices, about ¼ inch thick. Carefully slice the steak across the grain for a more tender cut. Next, toss the slices of meat in the cornstarch. Dispose of the excess cornstarch.
Heat the oil in the Instant Pot on Sauté. Add the flank steak and cook until slightly browned on both sides, about 2 minutes. Transfer the cooked steak to a paper towel-lined plate to drain excess grease.
Then add the ginger and garlic to the hot Instant Pot and sauté for about 1 minute. Add the water, soy sauce, beef broth, and brown sugar. Stir well and let simmer for about 5 minutes, allowing the sauce to thicken.
Hit Cancel on the Instant Pot to end the sauté mode. Transfer the flank steak back to the Instant Pot. Set the Instant Pot to Sealing Position and cook on Manual High Pressure for 10 minutes. Carefully set your Instant Pot to Venting Position to allow for a Quick Release once the Instant Pot is finished cooking.
Serve the Mongolian beef immediately on a bed of white rice, sprinkled with sesame seeds, and thinly sliced green onions.
Technically, you could substitute flour for cornstarch when searing the flank steak in this recipe, but we suggest that you use cornstarch for better results. The cornstarch will not only help produce a thicker sauce but develop a crisper crust on your steak and help seal in the juices as it sears in the Instant Pot.